Method For The Nondestructive Material Testing Of Highly Pure Polycrystalline Silicon

ABSTRACT

Noncontaminating and nondestructive testing of a shaped polysilicon body for a material defect is accomplished by exposing the shaped polysilicon body to ultrasound waves, and the ultrasound waves are registered by an ultrasound receiver after they have passed through the shaped polysilicon body or reflected therein, so that material defects in the polysilicon are detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the nondestructive materialtesting of highly pure polycrystalline silicon.

2. Background Art

Highly pure polycrystalline silicon, referred to below as polysilicon,is used inter alia as a starting material for the production ofelectronic components and solar cells. It is produced on an industrialscale by thermal decomposition and CVD deposition of a gas containingsilicon or a gas mixture containing silicon in Siemens reactors. Thepolysilicon is in this case formed as shaped polysilicon bodies. Thesecan subsequently be processed mechanically.

These shaped polysilicon bodies must be tested by means of anondestructive test method to assess their material quality. Acousticresonance analysis, also known as a “sound test”, is generally used forthis. The shaped polysilicon body is externally excited, for example bya gentle hammer blow, and the natural resonances resulting from thisprovide the person skilled in the art with information about thematerial quality of the shaped polysilicon body. One advantage ofresonance is the very short test time of only a few seconds. The entirespecimen body is furthermore studied during the test, i.e. the test is avolume-oriented test method. A disadvantage of resonance analysis isthat accurate localization or material defect identification is notpossible with this method. Moreover, the shaped polysilicon body istouched by a hammer blow and therefore contaminated in each test, whichnecessitates a subsequent cleaning step. Another disadvantage ofresonance analysis is that the shaped polysilicon body may be damaged bythe test. For example, superficial dislocations or even destruction ofthe shaped polysilicon body may take place. A further disadvantage isthat the shaped polysilicon bodies differ in their shape, for example indiameter and length or in length, width and height or finished articlegeometry, and each shaped polysilicon body generates a different naturalresonance owing to its different geometry. This makes comparison of thetest results more difficult. Small defects which compromise the materialquality, such as cracks, cavities or inclusions whose dimension is onlya few millimeters large, cannot be detected by this test method.

Visual inspection is another nondestructive test method for polysilicon.The entire surface of the shaped polysilicon body to be tested is inthis case assessed by a person skilled in the art. Visual inspection canbe improved by various aids, such a special illumination systems ormagnifying glasses. Although surface defects can also be identified andlocalized by visual inspection, here again material defects inside theshaped polysilicon bodies disadvantageously cannot be identified. Thetest is furthermore carried out by a person, i.e. the results are notobjective and reliably reproducible; rather, they depend on the tester's“daily form” and experience.

For both said nondestructive test methods, furthermore, the shapedpolysilicon body is handled with aids by the user. These aids, forexample gloves, may become laden with dirt particles between two tests,which leads to contamination of the shaped polysilicon body during thetest and necessitates a subsequent cleaning step.

SUMMARY OF THE INVENTION

It was an object of the invention to provide a nondestructive testmethod for a shaped polycrystalline silicon body to assess materialdefects, which does not have the disadvantages mentioned for the priorart. These and other objects are achieved by a method in which theshaped polycrystalline silicon body is exposed to ultrasound waves andthe ultrasound waves are registered by an ultrasound receiver after theyhave passed through the shaped polysilicon body, so that materialdefects in the polysilicon are detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a device for horizontal testing by means of oneembodiment of a method according to the invention in a side view andplan view;

FIG. 2 illustrates a device for vertical testing by means of oneembodiment of a method according to the invention in a side view andplan view. The numbering corresponds to FIG. 1;

FIG. 3 illustrates one embodiment of a test head unit (8) on a scanningarm (7) consisting of test head holders (11) and ultrasound test heads(12) for underwater testing, as described for example in Ex. 1; and

FIG. 4 illustrates one embodiment of a test head unit (8) on a scanningarm (7) consisting of the test head holders (11) and the ultrasound testheads (12) for water jet testing, as described for example in Ex. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

According to the invention, ultrasound waves in the frequency range offrom 100 kHz to 25 MHz, more preferably from 0.8 MHz to 20 MHz, andespecially 2-12 MHz, are introduced by an ultrasound test head into thepolycrystalline silicon. They propagate in a straight line in thepolysilicon, but are reflected at interfaces such as those found atmaterial defects (for example cracks, cavities or inclusions) but alsoat the transition from polycrystalline silicon to air. The materialdefects can be located best when their main dimension extendsperpendicularly to the propagation direction of the ultrasound waves inthe polysilicon. The shaped polysilicon body is therefore preferablyirradiated from all sides in the course of the method according to theinvention, since accurate noncontaminating and nondestructiveidentification of the position of the detected material defect in theshaped polysilicon body is thereby possible.

The shaped polysilicon body may be irradiated by means of the acoustictransmission method or by means of the pulse-echo method, the latterbeing preferred.

In the acoustic transmission method, the polycrystalline silicon isarranged between an ultrasound emitter and an ultrasound receiver. Theultrasound waves having passed through the polysilicon are convertedback into electrical oscillations (piezo effect) by an ultrasoundreceiver and are displayed. Owing to its interface with the polysilicon,a material defect is manifested by a reduced or missing signal. Depthdetermination of a defect is not possible with this method. Althoughthis variant of ultrasound measurement is usable in principle, thepulse-echo method as described below is nevertheless preferably employedaccording to the invention. Except the comments about the depthdetermination and defect evaluation of a defect, however, the followingcomments about the pulse-echo method also apply similarly for theacoustic transmission method.

In the pulse-echo method, the ultrasound test head is used as an emitterand receiver for the ultrasound waves. A sound pulse, which lies in thefrequency range of from 100 kHz to 25 MHz, more preferably from 0.8 MHzto 20 MHz, and especially 2-12 MHz, is radiated into the shapedpolysilicon body by the ultrasound test head, registered by the sameultrasound test head after complete or partial reflection and convertedback into a receiver pulse. The emitted pulse, backwall echo andpossibly defect echo(es) are registered electronically, depthdetermination of a material defect being possible using the respectivetime of flight of the reflected ultrasound waves. Any echo which occursin the defect expectation range, i.e. the range between the surface echoand the backwall echo, is then preferably categorized as a defect echo.The registered sound pulses are preferably output in dB (logarithm tothe base 10).

In the method according to the invention, preferably from 1 to 5ultrasound test heads are integrated in a test head holder. Theultrasound test heads and the corresponding test head holder will bereferred to below as a test head unit. Angled test heads with incidenceangles of between 10° and 85° in the silicon or perpendicular test headsare preferably used.

The test head unit is preferably moved up to the shaped polysiliconbody. To this end, the test head unit is preferably provided with acontactless spacer. The test head distance is preferably from 5 to 200mm, more preferably from 5 to 80 mm.

Utrasound coupling has been carried out with gel, oil, paste or water.Owing to the high material purity of the polysilicon, only water can beused according to the invention as a coupling medium for polysilicon.

Tests have shown that the following surface metal values are to be foundon the polysilicon surface after testing with drinking water as acoupling medium:

Fe Cr Ni Na Zn Cl Cu Mo Ti W K Co Mn Ca Mg V 1500 100 50 2500 100 300 501 250 1 500 0.5 15 4500 1500 0.5

All specifications are in pptw. The effect of this is that all thetested material must subsequently also be cleaned.

Surprisingly, the ultrasound coupling can also be carried out by meansof bubble-free, fully deionized water. Tests have shown that thefollowing surface metal values are to be found on the polysiliconsurface after testing with fully deionized water (pH≦7.0; resistance=18megOhm; free of suspended matter) as a coupling medium:

Fe Cr Ni Na Zn Cl Cu Mo Ti W K Co Mn Ca Mg V 15 1 0.5 25 10 30 1 1 25 15 0.5 0.5 45 11 0.5All specifications are in pptw. By using fully deionized water, it isthus possible to obviate subsequent recleaning of the shaped polysiliconbody.

The method according to the invention thus for the first time makes itpossible to test a shaped polysilicon body, the surface of the shapedpolysilicon body remaining highly pure (surface metal values: Fe≦15;Cr≦1; Ni≦0.5; Na≦25; Zn≦10; Cl≦30; Cu≦1; Mo≦1; Ti≦25; W≦1; K≦5; Co≦0.5;Mn≦0.5; Ca≦45; Mg≦11; V≦0.5; all specifications are in pptw).

The ultrasound coupling may be carried out by a water jet technique orby an immersion technique. With the water jet technique, the ultrasoundcoupling is preferably carried out by means of a water jet which joinsthe ultrasound test head to the surface of the shaped polysilicon bodyfreely from air bubbles. With the immersion technique, the entire testtakes place under water so that the ultrasound test head is likewisejoined bubble-free to the surface of the test body.

After the ultrasound coupling, the exposure of the shaped polysiliconbody to the ultrasound waves begins. The shaped polysilicon body ispreferably monitored by the ultrasound test head during the test. Themonitoring is carried out in any way, more preferably in the directionof the longitudinal axis of the shaped body to be tested, mostparticularly preferably in a lateral direction along the longitudinalaxis and the circumference of the shaped body to be tested.Simultaneously or alternatively, the shaped polysilicon body itself maybe moved, for example lowered, raised or moved horizontally. The shapedpolysilicon body itself may also be set in rotation.

The monitoring/test rate preferably lies between 1 and 1500 mm/s, morepreferably between 150 mm/s and 600 mm/s.

The signal evaluation of the reflected ultrasound waves is preferablycarried out in a computation instrument. In this case the signal of thereflected ultrasound waves in a defined time window, the so-calleddefect expectation range, is compared in the computation device with abase noise level or a defined signal threshold value. If the base noiselevel or the defined signal threshold value is exceeded, then the shapedpolysilicon body is categorized as defective and assigned to thedefective subset (shaped polysilicon body not ok). By varying the signalthreshold value, the sensitivity of the defect identification can bevaried continuously.

The result is in this case preferably output by means of a displaydevice which outputs an unambiguous result, generally “shapedpolysilicon body ok” or “shaped polysilicon body not ok”. The testparameters, such as the defined signal threshold value, start of thedefect expectation range, end of the defect expectation range, arepreferably stored in the form of test programs in the computation unitso that the method can be adapted rapidly and simply to various defectlimits for different dimensions and quality changes, depending on thegeometry and intended purpose of the shaped polysilicon body.Furthermore, as described, the computation instrument makes it possibleto determine the position of the defect in the shaped polysilicon bodyand thus specify defective regions in the shaped polysilicon body. Tothis end, the shaped silicon body with the defective and defect-freeregions may be represented on the result output.

After the ultrasound test, the shaped polysilicon is preferably dried,preferably by a nozzle supplied with compressed air which travels alongthe test path in the opposite direction to the test, until the specimenis dried.

The shaped polysilicon body preferably comprises polysilicon rods or rodpieces. The shaped polysilicon body preferably has a diameter of from 3mm to 300 mm, more preferably a diameter of from 50 mm to 200 mm. Thelength of the shaped body is in principle unrestricted, shaped bodieswith a length of from 10 mm to 4500 mm capable of being analyzed, inparticular from 100 mm to 3000 mm, preferably being tested. The shapedbodies are preferably provided with a machine-readable identity (ID)number, which makes it possible to record the shaped body with the aidof the ID number in the method according to the invention. This may, forexample, be done using the computation instrument.

The method is suitable for testing shaped polysilicon bodies of anygeometry. For example, even a banana-shaped body may be tested. Themethod furthermore permits nondestructive material testing of highlypure silicon. Concealed material defects in the surface vicinity andinside the material are detected. The method delivers an unambiguous andreproducible test result. When using the pulse-echo method, the materialdefects can also be accurately localized and identified inside thepolysilicon.

The method operates freely from contamination. The shaped polysiliconbody to be tested comes in contact only with the aid, i.e. water,preferably with fully deionized water (pH≦7.0; resistance≧0.5 megOhm,more preferably resistance≧18 megOhm; free of suspended matter).Preferably, this aid required for the test is continuously testedqualitatively to 100%, since the quality of this aid has a direct effecton the surface metal values of the shaped polysilicon body.

The method is a volume-oriented test method, i.e. material defects froma few tenths of a millimeter to millimeters can be detected andlocalized accurately throughout the shaped polysilicon body. Thus, evendefects with a diameter of 0.2 mm at a depth of 130 mm could be detectedin a shaped polycrystalline polysilicon body by means of the methodaccording to the invention. Conversely, only material defects in therange of centimeters to decimeters lying in the bulk can be detected bymeans of resonance analysis. Visual inspection allows onlysurface-oriented testing, i.e. only visible surface defects can bedetected.

The automated test method of the invention excludes error-pronesubjective assessment by a person, and it does not overlook any defects.The test method delivers an unambiguous and reproducible test result(specimen—OK/specimen—not OK), and it furthermore makes it possible todefine specification noncompliant regions of a shaped polysilicon body.The method does not need any special preparation of the shapedpolysilicon body to be tested, so that it can be incorporated simplyinto an existing fabrication process. For example, cylindrical andconical shaped polysilicon bodies for an FZ refining process can beinspected for material defects by the method. Rods or rod pieces (forexample cut rods, rebatch rods etc.) for an FZ or CZ refining processcan furthermore be studied for material defects.

The invention thus for the first time provides a shaped polysilicon bodythat contains no defects, which are preferably intended to mean cracks,cavities or inclusions with a projection surface larger than 0.03 mm². Ashaped polysilicon body according to the invention preferably containsno defects at all and exhibits a reduced cleaving and flaking behaviorduring the subsequent melting and refining processes.

Two embodiments of the method according to the invention areschematically represented in FIGS. 1 and 2.

The following examples serve to explain the invention further.

EXAMPLE 1 Horizontal Testing Under Water

A shaped polysilicon body (1) with a diameter of 200 mm and a rod lengthof 2500 mm is tested horizontally in a device according to FIG. 1 underwater. To this end the shaped polysilicon body (1) is arrangedhorizontally in the water reception trough (4) and clamped between thetips of the specimen retainers (2) and (3). The water reception trough(4) is filled with fully deionized water. In parallel with this, theshaped silicon body (1) provided with a machine-readable identity (ID)number is registered with the computation instrument (5+6), i.e. themachine-readable identity (ID) number is communicated to the computationinstrument (5+6). The test program stored in the computer instrument issubsequently selected and the test is started.

The scanning arm (7), at the end of which the ultrasound test head unit(8) is fastened, moves with a rate of advance of 10 mm/s in thedirection of the shaped polysilicon body (1). When the ultrasound testhead unit (8) reaches a distance of 5 mm between the ultrasound testhead unit (8) and the shaped polysilicon body (1), the advance of thescanning arm (7) is set.

The ultrasound test head unit (8) consists of 3 test head holders (11),4 ultrasound test heads (12) (FIG. 3) and the contactless distanceelectronics, which are integrated in the scanning arm (7) (not depicted)and keep the distance between the test head holders and the shaped bodyconstant.

The ultrasound test heads (12) are operated using the pulse-echo method.Each ultrasound test head (12) emits pulses with a frequency of 12 MHzin a definitively preset sequence and receives the reflected signals.The scanning arm (7) moves at a constant speed of 1200 mm/s from thespecimen retainer (2) to the specimen retainer (3) along the surface ofthe shaped polysilicon body (1).

The shaped polysilicon body (1) is rotated through 1 mm in thecircumferential direction by means of the specimen retainers (2) and(3). The scanning arm (7) moves back at 1200 mm/s from the specimenretainer (3) to the specimen retainer (2) along the surface of theshaped polysilicon body (1). The received signals are evaluated inparallel with this in the computation instrument (5+6) and the resultsare visualized. If a material defect is discovered, then a light (9)flashes until the end of the test. The described procedure is continuedwith the shaped polysilicon body (1) until the entire circumference ofthe shaped polysilicon body has been scanned and therefore tested. Afterthe end of the test, the fully deionized water is discharged from thewater reception trough (4). The specimen retainers (2) and (3) areopened and the shaped polysilicon body (1) is removed from the measuringdevice. Depending on the display (9), the shaped polysilicon body (1) iscategorized as defect-free or defective.

EXAMPLE 2 Horizontal Testing with Water Jet Coupling

A shaped polysilicon body (1) with a diameter of 200 mm and a rod lengthof 2500 mm is tested horizontally according to FIG. 1 with water jetcoupling. The test procedure is carried out similarly as Example 1, withthe difference that the water reception trough (4) is not filled withfully deionized water during the ultrasound testing and the ultrasoundtest head unit (8) is constructed as represented in FIG. 4 and asdescribed below:

The ultrasound test head unit (8) consists of 3 test head holders (11),4 ultrasound test heads (12) and the contactless distance electronics(not depicted), which are integrated in the scanning arm (7) and keepthe distance between the ultrasound test head unit (8) and the shapedpolysilicon body (1) constant. Fully deionized water is suppliedcontinuously to the test head holders (11) via lines (13). This waterfills the space around the ultrasound test heads (12) and forms a waterjet, which provides the ultrasound coupling between the ultrasound testheads (12) and the surface of the shaped polysilicon body (1). The testis carried out as described in Ex. 1.

Following the test run, but in the opposite direction, the shapedpolysilicon body (1) is dried by means of compressed air (not shown).After the shaped polysilicon body (1) has been dried, the tips of thespecimen retainers (2) and (3) are opened and the shaped polysiliconbody (1) is removed. Depending on the display (9), the shapedpolysilicon body (1) is categorized as defect-free or defective.

EXAMPLE 3 Vertical Testing Under Water

The test is carried out similarly to Example 1 by using the ultrasoundtest head unit represented in FIG. 3, with the difference that theshaped polysilicon body (1) is tested vertically as represented in FIG.2.

EXAMPLE 4 Vertical Testing with Water Jet Coupling

The test is carried out similarly to Example 2 by using the ultrasoundtest head unit represented in FIG. 4, with the difference that theshaped polysilicon body (1) is tested vertically as represented in FIG.2.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method for the noncontaminating and nondestructive testing of ashaped polysilicon body for a material defect, wherein the shapedpolysilicon body is exposed to ultrasound waves and the ultrasound wavesare registered by an ultrasound receiver after they have passed throughthe shaped polysilicon body, such that material defects in thepolysilicon are detected.
 2. The method of claim 1, wherein theultrasound waves are radiated through the shaped polysilicon body by anultrasound emitter and the ultrasound waves having passed through thepolysilicon are converted back into electrical oscillations by anultrasound receiver and are displayed.
 3. The method of claim 1, whereinthe ultrasound waves are radiated through the shaped polysilicon body byan ultrasound test head and, having been registered by the sameultrasound test head after complete or partial reflection, are convertedback into a receiver pulse.
 4. The method of claim 1, wherein thetransmitted and received ultrasound waves are electronically registeredand evaluated, a position determination of the material defect in theshaped polysilicon body being carried out using the respective time offlight of the ultrasound waves.
 5. The method of claim 1, wherein theultrasound test head is moved up to the shaped polysilicon body andcoupled to the shaped polysilicon body by means of ultrasound coupling,before exposure to the ultrasound waves takes place.
 6. The method ofclaim 1, wherein the signal evaluation of the reflected ultrasound wavesis carried out in a computation instrument, the time differences and/orthe signal strength between the reflected ultrasound waves beingcompared with predetermined parameters in the computation instrument. 7.The method of claim 1, wherein the ultrasound test head is moved up to atest head distance of from 5 to 200 mm from the shaped polysilicon body.8. The method of claim 1, wherein the ultrasound test head is moved upto a test head distance of from 5 to 80 mm from the shaped polysiliconbody.
 9. The method of claim 1, wherein the ultrasound coupling iscarried out by means of bubble-free water.
 10. The method as claimed inclaim 9, wherein the test is carried out by an immersion technique or bya water jet technique.
 11. The method as claimed in claim 8, wherein thewater is fully deionized (pH≦7.0; resistance≧0.5 megOhm.
 12. The methodas claimed in claim 8, wherein the water is fully deionized (pH≦7.0;resistance≧18 megOhm.
 13. The method of claim 1, wherein the surface ofthe shaped polysilicon body remains highly pure, with surface metalvalues: Fe≦15; Cr≦1; Ni≦0.5; Na≦25; Zn≦10; Cl≦30; Cu≦1; Mo≦1; Ti≦25;W≦1; K≦5; Co≦0.5; Mn≦0.5; Ca≦45; Mg≦11; V≦0.5; all specifications inpptw).
 14. The method of claim 1, wherein the shaped polysilicon bodycontains no undetectable defects which are larger than 0.03 mm² inprojection surface.
 15. A shaped polysilicon body which contains nodefects with a projection surface larger than 0.03 mm², as determined bythe method of claim 1.